Method for enhancing strength and hardness of powder metallurgy stainless steel

ABSTRACT

A method for enhancing strength and hardness of powder metallurgy stainless steels comprises steps of fabricating a stainless steel powder into a green compact; placing the green compact in a reducing environment and maintaining the green compact at a sintering temperature to form a sintered body; and placing the sintered body in a carbon-bearing atmosphere and maintaining the sintered body at a carburizing temperature below 600° C. to implant carbon atoms into the sintered body and form carburized regions in the sintered body. Thereby, the strength and hardness of powder metallurgy stainless steels can be improved. As the carburizing temperature is lower than 600° C., chromium would not react with carbon. Therefore, the strength and hardness of powder metallurgy stainless steels can be enhanced and the superior corrosion resistance is still preserved.

FIELD OF THE INVENTION

The present invention relates to a method for enhancing strength and hardness of powder metallurgy stainless steels, particularly to a method for carburizing a powder metallurgy stainless steels product to improve the strength and hardness thereof.

BACKGROUND OF THE INVENTION

Powder metallurgy has been extensively used to fabricate various metallic products. In the conventional powder metallurgy technologies, the metal powders to be sintered are mixed uniformly beforehand with lubricants. Next, the mixture of the metal powders is compacted into a green compact. Next, the green compact is heated to a high temperature and sintered at the high temperature for a period of time. Thereby, interdiffusion of atoms in the particles occurs and forms a sintered body.

In addition to the abovementioned powder-compaction process, a Metal Injection Molding (MIM) process, which integrates the powder metallurgy process and the plastic injection molding process, was proposed to fabricate mechanical parts having complicated shapes and demanding superior mechanical properties. In the MIM process, metal powders are mixed with binders to form a feedstock, and an injection molding machine is used to inject the feedstock into a mold to form a green compact. The green compact is debinded and then sintered at a high temperature to obtain a sintered body.

The stainless steel materials fabricated in the abovementioned powder metallurgy (including press-and-sinter and MIM) process may be categorized into the high-density stainless steel materials and the porous low-density stainless steel. Both types of sintered stainless steels are usually soft, which limits the applications. Normally, the surface hardness of the high-density stainless steel can be improved via a work-hardening method, such as rolling, forging, or other cold working methods. However, the abovementioned hardening method is unsuitable for a powder metallurgy sintered body because it is a net-shape process. Therefore, the industries usually use methods of chromium plating or shot-peening to increase the surface hardness of a powder metallurgy sintered body. However, the chromium coating is poor in adhesion and can be easily peeled of Shot-peening can only increase surface hardness by a limited extent. For parts with complicated shapes or small workpieces, some regions are difficult to be shot-peened. Therefore, shot-peening cannot increase hardness uniformly. Besides, there is no available method to improve the strength and hardness of porous stainless steel materials.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to increase the strength and hardness of sintered stainless steels and to overcome the problems of chromium-plating and shot-peening, which are the existing methods of improving the hardness and strength of high-density powder metallurgy stainless steels: chromium coating is easily peeled off, and shot-peening cannot increase hardness uniformly. The present invention also intends to improve hardness and strength of porous powder metallurgy stainless steels.

To achieve the abovementioned objectives, the present invention proposes a method for enhancing strength and hardness of powder metallurgy stainless steels, which comprises steps of: fabricating stainless steel powders into a green compact; sintering the green compact at a sintering temperature to form a sintered body; and placing the sintered body in a carbon-bearing atmosphere and maintaining the sintered body at a carburizing temperature below 600° C. to implant carbon atoms into the sintered body and form carburized regions.

The method for enhancing strength and hardness of powder metallurgy stainless steels of the present invention can achieve the following efficacies:

1. For high-density powder metallurgy stainless steels, the carburized regions have high concentration of carbon and provide high hardness for the surface of a sintered body. As carbon atoms implant into the sintered body uniformly, the surface hardness is evenly increased. Further, the carburized region is exempted from peeling off.

2. For porous powder metallurgy stainless steels, the carburized regions spread into the core of a sintered body. Thereby, the surface hardness and core hardness of the sintered body are obviously increased.

3. The carburized regions are formed at a temperature lower than 600° C. in the present invention. Thus, chromium will not react with carbon to form chromium carbide. Therefore, the present invention can increase strength and hardness of powder metallurgy stainless steels with the superior corrosion resistance still being preserved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for enhancing strength and hardness of powder metallurgy stainless steels according to one embodiment of the present invention;

FIG. 2 is an optical microscopic image of the microstructure of a sample used in Embodiment I;

FIG. 3 is an optical microscopic image of the microstructure of a sample used in Embodiment IX;

FIG. 4 is an optical microscopic image of the microstructure of a sample used in Embodiment X;

FIG. 5 is an optical microscopic image of the microstructure of a sample used in Embodiment XV;

FIG. 6 is an optical microscopic image of the microstructure of a sample used in Embodiment XVI;

FIG. 7 is an optical microscopic image of the microstructure of a sample used in Embodiment XVIII; and

FIG. 8 is an optical microscopic image of the microstructure of a sample used in Embodiment XIX.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention are described in detail in cooperation with drawings below.

Refer to FIG. 1 a flowchart of a method for enhancing strength and hardness of powder metallurgy stainless steels according to one embodiment of the present invention. In step S1, stainless steel powder is provided and fabricated into a green compact. The stainless steel powder is an iron-based material containing less than 2.0 wt % carbon, less than 1.0 wt % silicon, less than 2.0 wt % manganese, 12.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper. In one embodiment, the stainless steel powder is preferred to meet the chemical composition of 316L, 304L, 440C, or 17-4PH stainless steel regulated in the AISI (American Iron and Steel Institute) standard. The green compact is obtained via compacting stainless powder (press-and-sinter method) or molding stainless steel feedstock (MIM).

In Step S2, the green compact is placed in a reducing environment and maintained at a sintering temperature to sinter the green compact into a sintered body. The reducing environment may be vacuum environment or hydrogen-bearing atmosphere. The hydrogen-bearing atmosphere is preferred to have greater than 5.0 vol % of hydrogen. The sintering temperature ranges from 1,050 to 1,400° C. In Step S2, the process can be undertaken in an atmosphere sintering furnace or a vacuum furnace. After the green compact is placed in the atmosphere sintering furnace, a gas mixture of hydrogen and nitrogen or cracked ammonia is supplied to the atmosphere sintering furnace, and the atmosphere sintering furnace is heated to the sintering temperature and binders or lubricants in the green compact are removed in this period; the green compact is maintained at the temperature for a predetermined interval of time to form a sintered body. Next, the atmosphere sintering furnace is cooled down to the ambient temperature. Then, the sintered body is taken out from the atmosphere sintering furnace. Alternatively, the green compact is placed in a vacuum furnace. The vacuum furnace is pumped to a given degree of vacuum, and the vacuum furnace is heated to the sintering temperature; the green compact is maintained at the temperature for a predetermined interval of time to form a sintered body. The predetermined interval of time ranges from 30 minutes to 3 hours. In one embodiment, the sintering temperature or the sintering time is controlled to make the sintered body have a relative density of over 95%, whereby the few pores are not interconnected. In one embodiment, the sintering temperature or the sintering time is controlled to make the sintered body have a relative density of 30-95% to obtain a porous structure having interconnected pores.

In Step S3, the sintered body is placed in a carbon-bearing atmosphere and maintained at a carburizing temperature to let carbon atoms implant into the surface of the sintered body to form a carburized region. The carburizing temperature is lower than 600° C. and preferably between 400 and 580° C. In the present invention, the carbon-bearing atmosphere will include carbon monoxide, methane, propane, or other carbon-containing gases. The sintered body is maintained at the temperature and carburized for a given interval of time; then the furnace is cooled down to the ambient temperature. The carburization time is preferably set to be 24 hours. When the sintered body has a relative density of over 95%, the carburized region is formed on the surface of the sintered body and has a thickness of 10-50 μm. When the sintered body has a relative density of 30-95%, the carburized region is spread all over the sintered body from exterior to interior. In the present invention, Step S2 might be undertaken in an atmosphere sintering furnace or vacuum furnace, and Step S3 might be undertaken in a carburizing furnace. Alternatively, Step S2 and Step S3 may be undertaken in the same furnace. For example, after Step S2 is completed, the sintered body is not taken out from the sintering furnace or vacuum furnace, and a carbon-bearing atmosphere is directly supplied to the same furnace to undertake Step S3.

Below, embodiments are used to demonstrate a method for enhancing strength and hardness of powder metallurgy stainless steels of the present invention. However, the embodiments are only to exemplify the present invention but not to limit the scope of the present invention. Table.1 lists the chemical compositions of the stainless steel powder used in the embodiments and comparisons. Composition 1 is a commercial 316L stainless steel powder having an average particle diameter of 12.1 μm. Composition 2 is a commercial 17-4PH stainless steel powder having an average particle diameter of 11.5 μm. Composition 3 is a commercial 440C stainless steel powder having an average particle diameter of 11.3 μm. Composition 4 is a commercial 316L stainless steel powder having an average particle diameter of 39.7 μm. Composition 5 is a commercial 304L stainless steel powder having an average particle diameter of 40.2 μm. Table.2 lists the compositions and fabrication conditions of the samples used in Embodiments I-XIV. Table.3 lists the compositions and fabrication conditions of the samples used in Embodiments XV-XXV. Table.4 lists the compositions and fabrication conditions of the samples used in Comparisons I-IX.

In preparing the samples of the embodiments and comparisons, a stainless steel powder is mixed with a specified proportion of a lubricant and a specified proportion of a binder, and the mixture is fabricated into a green compact with a powder compaction process or an MIM process. Next, the green compact is debinded to remove the lubricant and the binder. Alternatively, a stainless steel powder is directly placed in a mold and sintered in an uncompacted form to obtain a green compact. Next, the samples are sintered and carburized according to the fabrication conditions listed in Tables. 2-4. Then the density, hardness, strength, and corrosion resistance of sintered material and the thickness of the carburized layer are measured. In the embodiments and comparisons, the MIM process or the powder compaction process are used to exemplify the fabrication process. However, the present invention does not exclude those with other powder metallurgy processes. The density of a sintered body is obtained with the Archimedes method. The theoretical density of the sintered body is obtained via calculation. Then, the density and the theoretical density are used to work out the relative density of the sintered body. The sintered bodies of Embodiments I-XIV and Comparisons I-III have relative densities over 95%. The sintered bodies of Embodiments XV-XXV and Comparisons IV-IX have relative densities below 95%.

The mechanical property tests include the surface hardness and core hardness tests, measured using either a Vickers hardness tester or a Rockwell hardness tester. The tensile strength and elongation are also measured. The corrosion resistance is tested following the MPIF (Metal Powder Industries Federation) Standard 62 and a frequently-used salt-spray method in the present invention. In the MPIF Standard 62, the carburized workpieces are immersed in a 2% weight sulfuric acid solution for 24 hours. Then, the weight loss is measured. If the weight loss per square decimeter is less than 0.005 g, the workpiece is a qualified one and designated by O. If the weight loss per square decimeter is greater than 0.005 g, the workpiece is an unqualified one and designated by X. The carburized workpieces are also tested with the salt-spray method, wherein the carburized workpieces are placed in a mist of 5% weight sodium chloride solution and the surface is examined with naked eyes to determine the interval of time after which corrosion occurs. The thickness of the carburized region is measured via observing the microscopic images of the carburized workpieces. The mechanical properties and corrosion resistances of Embodiments I-XIV are listed in Table.5. The mechanical properties and corrosion resistances of Embodiments XV-XXV are listed in Table.6. The mechanical properties and corrosion resistances of Comparisons I-IX are listed in Table.7.

Embodiment I

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body 10 a. After being cooled, the sintered body 10 a is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 a has a relative density of 96% and has a microstructure shown in FIG. 2. It can be observed in FIG. 2 that the sintered and carburized body 10 a has a carburized region 11 a with a thickness of about 42 μm on the surface. The sintered body 10 a has a surface hardness of about HV801 and a core hardness of about HV118. The sintered body 10 a has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment II

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered and carburized body has a relative density of 96% and has a carburized region with a thickness of about 42 μm on the surface. The sintered body has a surface hardness of about HV801 and a core hardness of about HV118. The sintered and carburized body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment III

This embodiment adopts a stainless steel powder of Composition II and a mean particle size of 11.5 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,320° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered and carburized body has a relative density of 97% and has a carburized region with a thickness of about 11 μm on the surface. The sintered body has a surface hardness of about HV610 and a core hardness of about HV250. The sintered and carburized body has qualified corrosion resistance and can tolerate the salt spray test for 35 hours.

Embodiment IV

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. An MIM process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,280° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 40 μm on the surface. The sintered body has a surface hardness of about HV802 and a core hardness of about HV118. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment V

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. A powder-compaction process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,280° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 40 μm on the surface. The sintered body has a surface hardness of about HV802 and a core hardness of about HV118. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment VI

This embodiment adopts a stainless steel powder of Composition II and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. An MIM process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,280° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 97% and has a carburized region with a thickness of about 11 μm on the surface. The sintered body has a surface hardness of about HV610 and a core hardness of about HV250. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 35 hours.

Embodiment VII

This embodiment adopts a stainless steel powder of Composition III and a mean particle size of 11.3 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,280° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 98% and has a carburized region with a thickness of about 13 μm on the surface. The sintered body has a surface hardness of about HV602 and a core hardness of about HV320. The sintered body can tolerate the salt spray test for 20 hours. However, the sintered body is not examined with the corrosion resistance test of the MPIF Standard 62 in this embodiment.

Embodiment VIII

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. An MIM process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 41 μm on the surface. The sintered body has a surface hardness of about HV801 and a core hardness of about HV118. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment IX

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. An MIM process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,120° C. for 2 hours to form a sintered and carburized body 10 b. After being cooled, the sintered body 10 b is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered and carburized body 10 b has a relative density of 96% and has a microstructure shown in FIG. 3. It can be observed in FIG. 3 that the sintered and carburized body 10 b has a carburized region 11 b with a thickness of about 39 μm on the surface. The sintered body 10 b has a surface hardness of about HV810 and a core hardness of about HV140. The sintered and carburized body 10 b has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment X

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered at a sintering temperature of 1,350° C. for 2 hours in an atmosphere furnace filled with hydrogen to form a sintered and carburized body 10 c. After being cooled, the sintered body 10 c is taken out from the atmosphere furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered and carburized body 10 c has a relative density of 96% and has a microstructure shown in FIG. 4. It can be observed in FIG. 4 that the sintered and carburized body 10 c has a carburized region 11 c with a thickness of about 41 μm on the surface. The sintered body 10 c has a surface hardness of about HV800 and a core hardness of about HV120. The sintered and carburized body 10 c has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment XI

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered at a sintering temperature of 1,350° C. for 2 hours in an atmosphere furnace filled with cracked ammonia to form a sintered body. After being cooled, the sintered body is taken out from the atmosphere furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 40 μm on the surface. The sintered body has a surface hardness of about HV810 and a core hardness of about HV190. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment XII

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered at a sintering temperature of 1,350° C. for 2 hours in an atmosphere furnace filled with a gas mixture of 95 vol % N₂ and 5 vol % H₂ to form a sintered body. After being cooled, the sintered body is taken out from the atmosphere furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 41 μm on the surface. The sintered body has a surface hardness of about HV800 and a core hardness of about HV201. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment XIII

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. An MIM process is used to fabricate the powder consisting of the abovementioned particles into a green compact. The green compact is debinded and sintered at a sintering temperature of 1,120° C. for 2 hours in an atmosphere furnace filled with hydrogen to form a sintered body. After being cooled, the sintered body is taken out from the atmosphere furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 41 μm on the surface. The sintered body has a surface hardness of about HV801 and a core hardness of about HV118. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment XIV

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 400° C. for 24 hours. Thus, the sintered body has a relative density of 96% and has a carburized region with a thickness of about 20 μm on the surface. The sintered body has a surface hardness of about HV698 and a core hardness of about HV118. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Embodiment XV

This embodiment adopts a stainless steel powder of Composition IV and a mean particle size of 39.7 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body 10 d. After being cooled, the sintered body 10 d is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 d has a relative density of 86% and has a microstructure shown in FIG. 5. It can be observed in FIG. 5 that the sintered and carburized body 10 d has a carburized region 11 d (the white region). The sintered body 10 d has a macroscopic hardness of about HRB75, a surface hardness of about HV820 and a core hardness of about HV220. The sintered body 10 d has a tensile strength of about 520 MPa and an elongation of 20%. The sintered body 10 d has qualified corrosion resistance and can tolerate the salt spray test for 6 hours. From this embodiment, when it has lower relative density, the interconnecting pores would increase, and the diffusion of carbon atoms would be deeper. Therefore, the overall strength and hardness of the sintered body increase with the decrease of the relative density.

Embodiment XVI

This embodiment adopts a stainless steel powder of Composition IV and a mean particle size of 39.7 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body 10 e. After being cooled, the sintered body 10 e is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 e has a relative density of 92% and has a microstructure shown in FIG. 6. It can be observed in FIG. 6 that the sintered and carburized body 10 e has a carburized region 11 e (the white region). The sintered body 10 e has a macroscopic hardness of about HRB56, a surface hardness of about HV802 and a core hardness of about HV145. The sintered body 10 e has a tensile strength of about 421 MPa and an elongation of 36%. The sintered body 10 e has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XVII

This embodiment adopts a stainless steel powder of Composition IV and a mean particle size of 39.7 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 400° C. for 24 hours. Thus, the sintered body has a relative density of 90% and has a carburized region. The sintered body has a macroscopic hardness of about HRB63, a surface hardness of about HV680 and a core hardness of about HV141. The sintered body has a tensile strength of about 420 MPa and an elongation of 30%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XVIII

This embodiment adopts a stainless steel powder of Composition V and a mean particle size of 40.2 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered and carburized body 10 f. After being cooled, the sintered body 10 f is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 f has a relative density of 86% and has a microstructure shown in FIG. 7. It can be observed in FIG. 7 that the sintered and carburized body 10 f has a carburized region 11 f (the white region). The sintered body 10 f has a macroscopic hardness of about HRB74, a surface hardness of about HV811 and a core hardness of about HV245. The sintered body 10 f has a tensile strength of about 519 MPa and an elongation of 16%. The sintered body 10 f has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XIX

This embodiment adopts a stainless steel powder of Composition V and a mean particle size of 40.2 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body 10 g. After being cooled, the sintered body 10 g is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body 10 g has a relative density of 92% and has a microstructure shown in FIG. 8. It can be observed in FIG. 8 that the sintered and carburized body 10 g has a carburized region 11 g (the white region). The sintered body 10 f has a macroscopic hardness of about HRB53, a surface hardness of about HV802 and a core hardness of about HV144. The sintered body 10 g has a tensile strength of about 416 MPa and an elongation of 38%. The sintered body 10 g has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XX

This embodiment adopts a stainless steel powder of Composition V and a mean particle size of 40.2 μm, and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 400° C. for 24 hours. Thus, the sintered body has a relative density of 90% and has a carburized region. The sintered body has a macroscopic hardness of about HRB61, a surface hardness of about HV675 and a core hardness of about HV142. The sintered body has a tensile strength of about 435 MPa and an elongation of 30%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XXI

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm, and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 90% and has a carburized region. The sintered body has a hardness f about HRB73, a surface hardness of about HV804 and a core hardness of about HV183. The sintered body has a tensile strength of about 520 MPa and an elongation of 27%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours.

Embodiment XXII

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 5 μm. The abovementioned powders are placed in a mold without compaction and then sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 93% and has a carburized region thereinside. The sintered body has a macroscopic hardness of about HRB55, a surface hardness of about HV800 and a core hardness of about HV140.

Embodiment XXIII

This embodiment adopts a stainless steel powder of Composition I and a mean particle size of 12.1 μm. The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 78% and has a carburized region. The sintered body has a macroscopic hardness of about HRB98, a surface hardness of about HV821 and a core hardness of about HV250.

Embodiment XXIV

This embodiment adopts a stainless steel powder of Composition IV and a mean particle size of 39.7 μm, The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 50% and has a carburized region. The sintered body has a hardness of about HRH18, a surface hardness of about HV815 and a core hardness of about HV488.

Embodiment XXV

This embodiment adopts a stainless steel powder of Composition V and a mean particle size of 40.2 μm. The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. Thus, the sintered body has a relative density of 50% and has a carburized region. The sintered body has a hardness of about HRH16, a surface hardness of about HV818 and a core hardness of about HV482.

Comparison I

This comparison adopts a stainless steel powder of Composition I and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,350° C. for 2 hours to form a sintered body. The sintered body has a relative density of 96%, a surface hardness of about HV120, and a core hardness of about HV120. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 72 hours.

Comparison II

This comparison adopts a stainless steel powder of Composition II and uses an MIM process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,320° C. for 2 hours to form a sintered body. The sintered body has a relative density of 97%, a surface hardness of about HV258, and a core hardness of about HV262. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 35 hours.

Comparison III

This comparison adopts a stainless steel powder of Composition III and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,280° C. for 2 hours to form a sintered body. The sintered body has a relative density of 98%, a surface hardness of about HV320, and a core hardness of about HV320. The sintered body can tolerate the salt spray test for 20 hours. However, the sintered body is not examined with the corrosion resistance test of the MPIF Standard 62 in this comparison.

Comparison IV

This comparison adopts a stainless steel powder of Composition IV and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body. The sintered body has a relative density of 86%, a macroscopic hardness of about HRB25, a surface hardness of about HV132, and a core hardness of about HV135. The sintered body has a tensile strength of about 295 MPa and an elongation of 24%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours. The strength and hardness of the sample of this comparison are lower than those of the samples of Embodiments XV, XVI and XVII.

Comparison V

This comparison adopts a stainless steel powder of Composition V and uses a powder-compaction process to fabricate the stainless steel powder into a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,250° C. for 2 hours to form a sintered body. The sintered body has a relative density of 86%, a macroscopic hardness of about HRB27, a surface hardness of about HV135, and a core hardness of about HV138. The sintered body has a tensile strength of about 291 MPa and an elongation of 25%. The sintered body has qualified corrosion resistance and can tolerate the salt spray test for 6 hours. The strength and hardness of the sample of this comparison are lower than those of the samples of Embodiments XVIII, XIX and XX.

Comparison VI

This comparison adopts a stainless steel powder of Composition I. A cyclone separator is used to obtain particles having a diameter smaller than 5 μm from the abovementioned stainless steel powder. The abovementioned particles are placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. The sintered body has a relative density of 93%, a macroscopic hardness of about HRB42, a surface hardness of about HV118 and a core hardness of about HV122.

Comparison VII

This comparison adopts a stainless steel powder of Composition I. The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. The sintered body has a relative density of 78%, a macroscopic hardness of about HRB16, a surface hardness of about HV121, and a core hardness of about HV122.

Comparison VIII

This comparison adopts a stainless steel powder of Composition IV. The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. The sintered body has a relative density of 50%, a surface hardness of about HV110, and a core hardness of about HV115. The macroscopic hardness HRH of the sintered body is too low to be measured in this comparison.

Comparison IX

This comparison adopts a stainless steel powder of Composition V. The stainless steel powder is placed in a mold and processed with a non-compaction sintering process to form a green compact. The green compact is debinded and sintered in a vacuum furnace at a sintering temperature of 1,190° C. for 2 hours to form a sintered body. After being cooled, the sintered body is taken out from the vacuum furnace and placed in a carburizing furnace for carburization at a temperature of 500° C. for 24 hours. The sintered body has a relative density of 50%, a surface hardness of about HV112, and a core hardness of about HV113. The macroscopic hardness HRH of the sintered body is too low to be measured in this comparison.

In Embodiments I-XIV, when the sintered bodies have compact microstructures (the relative densities are greater than 95%), the surface hardnesses may be as high as HV810, and the thicknesses of the carburized regions may be as great as 42 μm, with the superior corrosion resistance of stainless steel still being preserved. In Embodiments XV-XXV, when the sintered bodies have porous microstructures (the relative densities are smaller than 95%), the surface hardnesses may be as high as HV810, and carbon atoms can diffuse into the cores of the sintered bodies. Thereby, not only the surface hardnesses are increased, but also the core strengths are also increased. Thus, the surface hardnesses of the sintered bodies may be increased to as high as HV821, and the core hardnesses may be increased to as high as HV482. Further, the tensile strengths of the sintered bodies are also obviously increased with the preservation of the superior corrosion resistance of stainless steel.

In conclusion, the present invention implants carbon atoms into a sintered body to form carburized regions in the sintered body, whereby the hardness and strength of the sintered body is obviously increased by the high concentration of carbon atoms. When the sintered body is a porous structure, carbon atoms can diffuse into the interior of the sintered body. Thus the surface hardness, the core hardness and even the tensile strength of the sintered body are improved. When the sintered body is a compact structure, carbon atoms diffuse into the surface of the sintered body to form a carburized region in the surface, whereby the surface hardness of the sintered body is increased. In comparison with the conventional chromium-plating method and shot-peening method, the present invention improves the strength and hardness of powder metallurgy stainless steels more effectively. As the carburized region is formed at a temperature of lower than 600° C., chromium would not react with carbon to form chromium carbide. Therefore, the present invention can increase the strength and hardness of powder metallurgy stainless steels with the preservation of superior corrosion resistance.

The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.

TABLE 1 Chemical Compositions Used in Embodiments and Comparisons (Weight Percentage) Serial Number C Si Mn Cr Mo Ni Cu Nb P S Fe Composition 1 0.025 0.80 0.85 16.40 2.10 12.62 0.03 0 0.015 0.008 balance Composition 2 0.030 0.82 0.82 15.74 0.01 4.27 3.26 0.30 0.018 0.008 balance Composition 3 1.020 0.84 0.81 16.96 0.11 0.16 0 1.53 0 0 balance Composition 4 0.025 0.43 1.98 16.40 2.22 13.26 0 0 0.018 0.008 balance Composition 5 0.028 0.52 1.99 17.52 0 8.85 0 0 0.015 0.008 balance

TABLE 2 Fabrication Conditions for Embodiments I-XIV Carbu- Serial Chemical Particle Fabrication Sintering Reducing Sintering Sintering C-containing rizing Number Composition size Method Temperature Environment Temperature Temperature Atmosphere Time Embodiment 1 Composition 1 12.1 μm MIM 1350° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 2 Composition 1 12.1 μm Compaction 1350° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 3 Composition 2 11.5 μm MIM 1320° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 4 Composition 1   5 μm MIM 1280° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 5 Composition 1   5 μm Compaction 1280° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 6 Composition 2   5 μm MIM 1280° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 7 Composition 3 11.3 μm Compaction 1280° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 8 Composition 1   5 μm MIM 1190° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 9 Composition 1   5 μm MIM 1120° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 10 Composition 1 12.1 μm MIM 1350° C. Hydrogen 2 Hours 500° C. CO 24 Hours Embodiment 11 Composition 1 12.1 μm MIM 1350° C. Cracked Ammonia 2 Hours 500° C. CO 24 Hours Embodiment 12 Composition 1 12.1 μm MIM 1350° C. Hydrogen + 2 Hours 500° C. CO 24 Hours Nitrogen Embodiment 13 Composition 1   5 μm MIM 1120° C. Hydrogen 2 Hours 500° C. CO 24 Hours Embodiment 14 Composition 1 12.1 μm MIM 1350° C. Vacuum 2 Hours 400° C. CO 24 Hours

TABLE 3 Fabrication Conditions for Embodiments XV-XXV Serial Chemical Particle Fabrication Sintering Reducing Sintering Sintering C-containing Carburizing Number Composition size Method Temperature Environment Temperature Temperature Atmosphere Time Embodiment 15 Composition 4 39.7 μm Compaction 1250° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 16 Composition 4 39.7 μm Compaction 1350° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 17 Composition 4 39.7 μm Compaction 1250° C. Vacuum 2 Hours 400° C. CO 24 Hours Embodiment 18 Composition 5 40.2 μm Compaction 1250° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 19 Composition 5 40.2 μm Compaction 1350° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 20 Composition 5 40.2 μm Compaction 1250° C. Vacuum 2 Hours 400° C. CO 24 Hours Embodiment 21 Composition 1 12.1 μm MIM 1250° C. Vacuum 2 Hours 500° C. CO 24 Hours Embodiment 22 Composition 1   5 μm Uncompacted 1190° C. Vacuum 2 Hours 500° C. CO 24 Hours Sintering Embodiment 23 Composition 1 12.1 μm Uncompacted 1190° C. Vacuum 2 Hours 500° C. CO 24 Hours Sintering Embodiment 24 Composition 4 39.7 μm Uncompacted 1190° C. Vacuum 2 Hours 500° C. CO 24 Hours Sintering Embodiment 25 Composition 5 40.2 μm Uncompacted 1190° C. Vacuum 2 Hours 500° C. CO 24 Hours Sintering

TABLE 4 Fabrication Conditions for Comparisons I-IX Serial Chemical Fabrication Sintering Reducing Sintering Sintering C-containing Carburizing Number Composition Method Temperature Environment Temperature Temperature Atmosphere Time Comparison 1 Composition 1 MIM 1350° C. Vacuum 2 Hours — — — Comparison 2 Composition 2 MIM 1320° C. Vacuum 2 Hours — — — Comparison 3 Composition 3 Compaction 1280° C. Vacuum 2 Hours — — — Comparison 4 Composition 4 Compaction 1250° C. Vacuum 2 Hours — — — Comparison 5 Composition 5 Compaction 1250° C. Vacuum 2 Hours — — — Comparison 6 Composition 1 Uncompacted 1190° C. Vacuum 2 Hours — — — Sintering Comparison 7 Composition 1 Uncompacted 1190° C. Vacuum 2 Hours — — — Sintering Comparison 8 Composition 4 Uncompacted 1190° C. Vacuum 2 Hours — — — Sintering Comparison 9 Composition 5 Uncompacted 1190° C. Vacuum 2 Hours — — — Sintering

TABLE 5 Mechanical Properties and Corrosion Resistances Obtained in Embodiments I-XIV Car- Surface Core burized Corrosion Serial Hard- Hard- Region Resistance Salt Number ness ness Thickness Test Spray Test Embodiment 1 HV 801 HV 118 42 μm ◯ 72 Hours Embodiment 2 HV 801 HV 118 42 μm ◯ 72 Hours Embodiment 3 HV 610 HV 250 11 μm ◯ 35 Hours Embodiment 4 HV 802 HV 118 40 μm ◯ 72 Hours Embodiment 5 HV 802 HV 118 40 μm ◯ 72 Hours Embodiment 6 HV 610 HV 250 11 μm ◯ 35 Hours Embodiment 7 HV 602 HV 320 13 μm — 20 Hours Embodiment 8 HV 801 HV 118 41 μm ◯ 72 Hours Embodiment 9 HV 810 HV 140 39 μm ◯ 72 Hours Embodiment 10 HV 800 HV 120 41 μm ◯ 72 Hours Embodiment 11 HV 810 HV 190 40 μm ◯ 72 Hours Embodiment 12 HV 800 HV 201 41 μm ◯ 72 Hours Embodiment 13 HV 801 HV 118 41 μm ◯ 72 Hours Embodiment 14 HV 698 HV 118 20 μm ◯ 72 Hours

TABLE 6 Mechanical Properties and Corrosion Resistances Obtained in Embodiments XV-XXV Serial Corrosion Number Hardness Surface Hardness Core Hardness Elongation Tensile Strength Resistance Test Salt Spray Test Embodiment 15 HRB 75 HV 820 HV 220 20% 520 MPa ◯ 6 Hours Embodiment 16 HRB 56 HV 802 HV 145 36% 421 MPa ◯ 6 Hours Embodiment 17 HRB 63 HV 680 HV 141 30% 420 MPa ◯ 6 Hours Embodiment 18 HRB 74 HV 811 HV 245 16% 519 MPa ◯ 6 Hours Embodiment 19 HRB 53 HV 802 HV 144 38% 416 MPa ◯ 6 Hours Embodiment 20 HRB 61 HV 675 HV 142 30% 435 MPa ◯ 6 Hours Embodiment 21 HRB 73 HV 804 HV 183 27% 520 MPa ◯ 6 Hours Embodiment 22 HRB 55 HV 800 HV 140 — — — — Embodiment 23 HRB 98 HV 821 HV 250 — — — — Embodiment 24 HRH 18 HV 815 HV 488 — — — — Embodiment 25 HRH 16 HV 818 HV 482 — — — —

TABLE 7 Mechanical Properties and Corrosion Resistances Obtained in Comparisons I-IX Serial Corrosion Number Hardness Surface Hardness Core Hardness Elongation Tensile Strength Resistance Test Salt Spray Test Comparison 1 — HV 120 HV 120 — — ◯ 72 Hours Comparison 2 — HV 258 HV 262 — — ◯ 35 Hours Comparison 3 — HV 320 HV 320 — — — 20 Hours Comparison 4 HRB 25 HV 132 HV 135 24% 295 MPa ◯  6 Hours Comparison 5 HRB 27 HV 135 HV 138 25% 291 MPa ◯  6 Hours Comparison 6 HRB 42 HV 118 HV 122 — — — — Comparison 7 HRB 16 HV 121 HV 122 — — — — Comparison 8 — HV 110 HV 115 — — — — Comparison 9 — HV 112 HV 113 — — — — 

1. A method for enhancing strength and hardness of powder metallurgy stainless steels, comprising steps of: fabricating a stainless steel powder into a green compact; placing the green compact in a reducing environment and maintaining the green compact at a sintering temperature to form a sintered body; and placing the sintered body in a carbon-bearing atmosphere and maintaining the sintered body at a carburizing temperature below 600° C. to implant carbon atoms into the sintered body and form carburized regions in the sintered body.
 2. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the reducing environment is vacuum or hydrogen-bearing atmosphere.
 3. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the sintering temperature ranges from 1,050 to 1,400° C.
 4. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the carburizing temperature ranges from 400 to 580° C.
 5. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the sintered body has a relative density of greater than 30%.
 6. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the green compact is fabricated with an Metal Injection Molding method.
 7. The method for enhancing strength and hardness of powder metallurgy stainless steel according to claim 1, wherein the green compact is fabricated with a powder-compaction method.
 8. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the carbon-bearing atmosphere is an atmosphere containing carbon monoxide, methane, or propane.
 9. The method for enhancing strength and hardness of powder metallurgy stainless steels according to claim 1, wherein the stainless steel powder is an iron-based material containing less than 2.0 wt % carbon, less than 1.0 wt % silicon, less than 2.0 wt % manganese, 12.0-19.0 wt % chromium, less than 15.0 wt % nickel, less than 6.0 wt % molybdenum, and less than 6.0 wt % copper. 